The invention generally relates to electronically controlled motors and, in particular, to commutating windings of such motors in response to a back EMF signal in the windings of such motors.
The back EMF signal inducted in an unused winding of a multiphase electric motor may be used to control commutation of another winding. For example, in a three phase electronically commutated motor, the back EMF signal in an unused winding may be used to turn off one winding and to turn on the other winding. The winding most recently turned off then becomes the unused winding so that its back EMF signal is used to turn on the next winding and turn off the other winding. The process repeats as the rotor rotates. For example, see co-assigned, co-invented U.S. Pat. Nos. 4,390,826; 4,449,079; 4,513,230; and 4,532,459; the entire disclosures of which are incorporated herein by reference.
In practice, a common technique is to integrate the back EMF signal in the unused winding relative to the motor neutral in order to produce the firing signal for turning on the next winding. A problem with this technique in electronically commutated motors, however, is that it is susceptible to integrator offsets, i.e., rotor position error, during operation of the motor at low speeds. These offsets are caused by mutually coupled switching noise from the energized winding. The integrator offsets can be large enough to cause starting problems.